Negative electrode for nonaqueous- electrolyte secondary battery and battery employing the same

ABSTRACT

100 parts by weight of a carbon material having irreversible capacity and 20 to 150 parts by weight of a lithium-containing complex nitride represented by the general formula Li 3-X M X N wherein M is at least one selected from the group consisting of Co, Ni, Mn and Cu, and wherein 0.2≦X≦0.8, are included in a negative electrode thereby to compensate for the irreversible capacity of the carbon material by the above-described nitride. This enables the maximum utilization of large capacity possessed by an amorphous carbon or low crystalline carbon, thereby making it possible to provide a non-aqueous electrolyte secondary battery having high capacity and excellent cycle reversibility.

TECHNICAL FIELD

[0001] The present invention mainly relates to a negative electrodeyielding a high-capacity non-aqueous electrolyte secondary battery withexcellent charge/discharge cycle characteristic.

BACKGROUND ART

[0002] At present, non-aqueous electrolyte secondary batteries such as alithium ion battery using, as the active material, a material capable ofreversibly absorbing and desorbing lithium ion have been put intopractical use.

[0003] For the positive electrode of the non-aqueous electrolytesecondary battery, for example, LiCoO₂, which is a lithium-containingcomplex oxide, is being employed. Li ions are originally contained inthe positive electrode, and the Li ions are reversibly absorbed in anddesorbed from a carbon material in the negative electrode during chargeand discharge.

[0004] Apart from LiCoO₂, lithium-containing complex oxides includeLiNiO₂, LiMn₂O₄ and complexes thereof. These oxides exhibit a potentialas high as about +4 V with respect to the potential of metallic lithiumand also have large reversible capacity. Therefore, they are excellentmaterials for use as active materials, capable of realizing batteries ofhigh voltage and high capacity.

[0005] On the other hand, for the negative electrode of the non-aqueouselectrolyte secondary battery, carbon materials are commonly used.Carbon materials are also capable of reversibly absorbing and desorbingLi ions. However, in the case of graphite, for example, the theoreticalupper limit for the amount of Li to be absorbed is the amount requiredfor formation of C₆Li, that is, one Li atom per six carbon atoms, andthe charge/discharge capacity thereof is 372 mAh/g.

[0006] Therefore, with the aim of achieving a further increase in thecapacity of the non-aqueous electrolyte secondary battery, many studieshave been undertaken on negative electrode materials. Among them is aproposal to improve carbon materials in order to achieve an increasedbattery capacity. For example, it has been reported that amorphouscarbons and low crystalline carbons have a capacity much higher than thetheoretical capacity of graphite (e.g., the 39th Battery SymposiumAbstract volume, pp. 443-444 (3D12)).

[0007] Although amorphous carbons and low crystalline carbons have largetheoretical capacity, they have the problem of having large irreversiblecapacity. The irreversible capacity refers to a capacity attributed to,of the Li ions absorbed in a carbon material, the Li ions which remaincaptured in the carbon material to be incapable of being desorbed duringthe subsequent discharge process and thus do not participate in thebattery reaction any longer.

[0008] When a carbon material has irreversible capacity, a portion of Liions which have been supplied to the carbon material in the negativeelectrode from a lithium-containing complex oxide in the positiveelectrode during the initial charge, is not able to return to thepositive electrode during the subsequent discharge. Even in such casewhere a carbon material having large theoretical capacity is employed,it is difficult to obtain a high-capacity battery if the material haslarge irreversible capacity.

[0009] As a countermeasure against the irreversible capacity of a carbonmaterial, an electrode formation process has been devised, by whichlithium in an amount corresponding to the irreversible capacity iselectrochemically charged into the carbon material in advance. Theelectrode formation process is excellent in that the formation can becontrolled according to the amount of current to be passed. However, itnecessitates charging an electrode once and using the electrode again toassemble a battery, resulting in complicated steps and low productivity.

[0010] As another countermeasure, a method of compensating for theirreversible capacity has been devised, which involves attachingmetallic lithium to the negative electrode to automatically allow Liions to move between the carbon material and the metallic lithium, whichare in a state of short-circuit with the electrode interposedtherebetween. In the case of this method, however, Li ions may notsufficiently move depending on the form of the electrode plate, so thatmetallic lithium remains in the negative electrode to cause variationsin performance, safety problems and the like.

[0011] For the reasons as set forth above, little progress has been madein the practical use of amorphous carbons and low crystalline carbons,despite of the fact that they are promising as the negative electrodematerial.

[0012] Therefore, there is a demand for effective techniques that can beused in place of the ones described above in order to compensate for theirreversible capacity of carbon materials.

DISCLOSURE OF INVENTION

[0013] The present invention offers an effective technique to compensatefor the irreversible capacity of a negative electrode containing acarbon material having high theoretical capacity.

[0014] More specifically, the present invention relates to a negativeelectrode for a non-aqueous electrolyte secondary battery comprising:100 parts by weight of a carbon material having irreversible capacityduring the initial charge and discharge; and 20 to 150 parts by weightof a lithium-containing complex nitride represented by the generalformula Li_(3-X)M_(X)N wherein M is at least one selected from the groupconsisting of Co, Ni, Mn and Cu, and wherein 0.2≦X≦0.8.

[0015] The lithium-containing complex nitride is preferablyLi_(3-X)Co_(X)N wherein 0.2≦X≦0.55.

[0016] The carbon material is preferably a low or less crystallinecarbon.

[0017] The low crystalline carbon is preferably in a fibrous form havinga mean fiber diameter of 1 to 50 μm and a mean fiber length of 10 to 200μm.

[0018] Alternatively, the carbon material is preferably an amorphouscarbon.

[0019] In the case where the carbon material is an amorphous carbon, thenegative electrode preferably contains 33 to 150 parts by weight of thelithium-containing complex nitride per 100 parts by weight of theamorphous carbon.

[0020] The present invention also related to a non-aqueous electrolytesecondary battery comprising: a positive electrode comprising alithium-containing complex oxide capable of absorbing and desorbinglithium ion; the above-described negative electrode; and a non-aqueouselectrolyte interposed between the positive electrode and negativeelectrode.

[0021] The present invention is effective for, for example, a negativeelectrode comprising a carbon material with a theoretical capacity ofnot less than 350 mAh/g. Such carbon materials include a low crystallinecarbon and an amorphous carbon.

[0022] The amorphous carbon refers to a carbon material which is notgraphitized even when sintered at a high temperature exceeding 1400° C.,and is generally called a hard carbon.

[0023] The low crystalline carbon refers to a carbon material preparedby intentionally sintering at a low temperature of from 700° C. to 1400°C., a material which is graphitized when sintered at a high temperatureexceeding 1400° C., and is generally called a soft carbon.

[0024] The low crystalline carbon and amorphous carbon have a specificsurface area of, for example, 0.5 to 2 m²/g. Accordingly, the lowcrystalline carbon and amorphous carbon are less likely to cause a sidereaction such as electrolyte decomposition.

[0025] The low crystalline carbon and amorphous carbon have a bulkdensity of, for example, about 1.0 g/cc. Therefore, the low crystallinecarbon and amorphous carbon have good volumetric efficiency.

[0026] On the other hand, acetylene black, which has conventionally beencontained in batteries, has a specific surface area of about 70 m²/g,bulk density of about 0.03 g/cc and theoretical capacity of not morethan 100 mAh/g.

BRIEF DESCRIPTION OF DRAWINGS

[0027]FIG. 1 is a vertical sectional view of a button-type test cell.

[0028]FIG. 2 is a graph showing the relation between thecharge/discharge voltages and capacities at 1st cycle and 5th cycle, ofthe test cell of Example 1.

[0029]FIG. 3 is a graph showing the relation between thecharge/discharge voltages and capacities at 1st cycle and 5th cycle, ofthe test cell of Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] In the present invention, the irreversible capacity of a carbonmaterial having large theoretical capacity such as an amorphous carbonor low crystalline carbon, is compensated for by Li ions in alithium-containing complex nitride.

[0031] The negative electrode of the present invention comprises acarbon material as the main negative electrode material, alithium-containing complex nitride as an auxiliary negative electrodematerial, and a binder. The lithium-containing complex nitridepreferably has high capacity as well as excellent reversibility incharge and discharge.

[0032] Techniques employing the lithium-containing complex nitride as anactive material for batteries are relatively new, and examples includethe following.

[0033] Japanese Unexamined Patent Publication No. Hei 7-78609 disclosesa lithium nitride-metal compound as an electrode material for anelectrochemical device. The lithium-containing complex nitride havinghigh capacity and excellent reversibility in charge/discharge isrepresented by the general formula: Li_(3-X)M_(X)N, which is obtained byreplacing a portion of Li in the lithium nitride Li₃N by a transitionmetal such as copper, iron, manganese, cobalt or nickel. The nitridewherein the value of X satisfies 0.2<X≦0.8 has particularly highcapacity.

[0034] The present inventors have found that a lithium-containingcomplex nitride represented by the general formula: Li_(3-X)Co_(X)N(0.3≦X≦0.55) wherein the transition metal is cobalt, has a high capacityof not less than 700 mAh/g as well as excellent reversibility in chargeand discharge.

[0035] The compensation of Li ions from the lithium-containing complexnitride to the carbon material in the negative electrode is performed byelectrochemical action in the presence of an electrolyte afterconstructing a battery.

[0036] The reaction potential of the lithium-containing complex nitrideand the reaction potential of the carbon material are different fromeach other, and therefore local cells are formed in the negativeelectrode. The carbon material has various reaction potentials. In thecase where the carbon material has a reaction potential higher than thereaction potential of the lithium-containing complex nitride, thepotential difference causes Li ions to automatically transfer from thelithium-containing complex nitride to the carbon material.Alternatively, in the case where the carbon material has a reactionpotential lower than the reaction potential of the lithium-containingcomplex nitride, the potential difference does not cause the automatictransfer of Li ions; however, Li ions are supplied from the positiveelectrode into the carbon material during the initial charge. Then, Liions in an amount corresponding to the irreversible capacity attributedto the Li ions incapable of returning to the positive electrode duringthe subsequent discharge, are supplied from the lithium-containingcomplex nitride to the positive electrode. Ultimately, the irreversiblecapacity is completely compensated for by the lithium-containing complexnitride.

[0037] After the initial discharge, the positive electrode has returnedto the initial discharged state in which Li ions are fully chargedtherein. Moreover, the carbon material in the negative electrode is in adischarged state in which Li ions in an amount corresponding to theirreversible capacity have already been compensated for, and thelithium-containing complex nitride is also in a discharged state inwhich Li ions in an amount corresponding to the above-describedirreversible capacity have been desorbed therefrom.

[0038] Therefore, the amount of the lithium-containing complex nitridecontained in the negative electrode is preferably equivalent to, orslightly greater than the amount corresponding to the irreversiblecapacity of the carbon material. Too large an amount of thelithium-containing complex nitride results in an increase in the amountof the nitride which does not participate in charge/discharge and thusis a disadvantage in increasing the battery capacity.

[0039] It is difficult to ascertain the irreversible capacity of acarbon material in an actual battery; however, according to studiesconducted by the present inventors, the most suitable amount of thelithium-containing complex nitride represented by the general formula:Li_(3-X)Co_(X)N (0.3≦X≦0.55) is 33 to 150 parts by weight per 100 partsby weight of an amorphous carbon in the case where the carbon materialis an amorphous carbon, and 20 to 150 parts by weight per 100 parts byweight of a low crystalline carbon in the case where the carbon materialis a low crystalline carbon.

[0040] As described above, the present invention makes it possible toutilize even a carbon material, which, despite having a great capacity,has hitherto been difficult to be put into practical use because of theirreversible capacity.

[0041] The negative electrode of the present invention is fabricated,for example, by applying to the surface of a negative electrode currentcollector, a mixture prepared by mixing a carbon material powder havinghigh capacity and irreversible capacity, a lithium-containing complexnitride powder and a binder resin.

[0042] Since the lithium-containing complex nitride is highly reactivewith water and hence suffers degradation from water, the solvent usedfor, for example, the preparation of the mixture is preferably a highlydehydrated one.

[0043] Since the amorphous carbon and low crystalline carbon have lowerelectronic conductivity compared to graphite, it is preferable to add aconductive agent into the above-described mixture to improve theconductivity of the negative electrode. As the conductive agent, anymaterial having excellent electronic conductivity may be used; forexample, artificial graphite, acetylene black or carbon fiber ispreferably used. However, since the carbon material andlithium-containing complex nitride as the negative electrode materialshave a certain degree of electronic conductivity, it is possible toconstruct a battery without addition of any conductive agent.

[0044] The amorphous carbon is produced, for example, by subjecting astarting material such as a phenol resin, epoxy resin or isotropic pitchto a curing process at 100 to 300° C. in air, followed by sintering at700 to 1400° C. in an inert atmosphere such as nitrogen or argon.

[0045] The low crystalline carbon is produced by sintering a pitch,polymer or the like at 700 to 1400° C. in an inert atmosphere. As thepitch, a petroleum pitch, coal tar pitch, mesophase pitch or the like ispreferably used, and polyimide or the like is preferably used as thepolymer.

[0046] The low crystalline carbon is preferably in a fibrous form havinga mean fiber diameter of 1 to 50 μm and a mean fiber length of 10 to 200μm. In the case where a fibrous carbon is employed, the fibrous carbonis dispersed between particles of the lithium-containing complex nitridein the mixture, and therefore the electronic conductivity between thenitride particles can be improved to enhance the battery performance.Moreover, it is possible to reduce the amount of any other conductiveagent for use in the negative electrode. Consequently, the contentproportion of the lithium-containing complex nitride in the negativeelectrode is increased, so that the capacity of the negative electrodecan also be increased.

[0047] In order to produce the fibrous low crystalline carbon, firstly,a starting material such as a mesophase pitch is heated at 100 to 300°C. to be softened, followed by a spinning process. Thereafter, a curingprocess may be performed on the fiber surface by heating the fiberobtained through the spinning process in an oxidizing atmosphere such asoxygen or air. By sintering the fiber thus obtained at 700 to 1400° C.in an inert atmosphere, it is possible to produce the fibrous lowcrystalline carbon.

[0048] As other components of the negative electrode used in the presentinvention, such as a negative electrode current collector,conventionally known components may be used.

[0049] The positive electrode used in the present invention preferablycomprises a lithium-containing complex oxide such as Li_(X)CoO₂,Li_(X)NiO₂ or Li_(X)MnO₂. Although the mean particle size of thelithium-containing complex oxide is not specifically limited, it ispreferably 1 to 30 μm.

[0050] As the positive electrode conductive agent, positive electrodecurrent collector and other components of the positive electrode used inthe present invention, conventionally known components may be used.

[0051] The non-aqueous electrolyte of the present invention comprises asolvent and a lithium salt dissolved in the solvent. In particular, itis preferable to use a non-aqueous electrolyte prepared by dissolvingLiPF₆ as a solute in a mixed solvent containing at least ethylenecarbonate and ethylmethyl carbonate. The concentration of the solute inthe non-aqueous electrolyte is preferably 0.2 to 2 mol/L, morepreferably 0.5 to 1.5 mol/L.

[0052] Additionally, any other compound may be added to the electrolyteas an additive in order to improve discharging and charge/dischargecharacteristics and the like.

[0053] Among the shapes of batteries are a coin type, button type, sheettype, laminated type, cylindrical type, flat type and prismatic type,and the present invention is applicable to any of these.

[0054] In the following, the present invention is described by referenceto examples.

EXAMPLE 1

[0055] A description is made on the case where an amorphous carbon wasused as a negative electrode active material.

[0056] The amorphous carbon was produced by sintering a polyamide resinat 2700° C. under an inert atmosphere.

[0057] In general, sintering a starting material such as a petroleumcoke at such a high temperature causes the crystal to grow to yieldgraphite. However, sintering a starting material such as a polyamideresin at a high temperature does not cause the crystal to grow, so thatan amorphous carbon is produced. The sintered material was pulverized tohave a mean particle size of about 15 μm and used in a powder form. TheX-ray diffraction pattern of this powder showed a broad profile, whichwas peculiar to amorphous carbons.

[0058] Next, the charge/discharge performance of this amorphous carbonwas confirmed in the following manner.

[0059] 100 parts by weight of powder of the amorphous carbon, 2 parts byweight at resin component of an aqueous dispersion of styrene butadienerubber (SBR) as a binder and an aqueous solution of 1 wt % carboxymethylcellulose (CMC) were mixed together and then sufficiently kneaded togive a mixture. This mixture was applied to a copper foil by using adoctor blade, dried and then rolled by means of a roller to form anelectrode sheet.

[0060] This electrode sheet was punched into a disc shape and then usedto fabricate a coin-type model cell having a counter electrodecomprising metallic lithium. The inside of the model cell was filledwith an electrolyte. The electrolyte was prepared by dissolving LiPF₆ ata concentration of 1 mol/L, in a mixed solvent containing ethylenecarbonate and ethylmethyl carbonate at a volume ratio of 1:1.

[0061] Then, the charge/discharge behavior and capacity of the modelcell were confirmed. The charging and discharging were performed at aconstant current of 0.3 mA/cm², with the end-of-discharge voltage set at1.5 V, and the end-of-charge voltage at 0 V.

[0062] The amorphous carbon absorbed Li ions in an amount correspondingto about 860 mAh/g during the initial charge; however, it desorbed Liions in an amount corresponding to only about 630 mAh/g during thesubsequent discharge, having an irreversible capacity of about 230mAh/g. Thereafter, the charging and discharging proceeded reversibly.

[0063] Next, a description is made on the case where Li_(2.5)Co_(0.5)Nhaving a low reaction potential and excellent reversibility was used asa lithium-containing complex nitride. The lithium-containing complexnitride was synthesized in the following manner.

[0064] Lithium nitride (Li₃N) powder and metallic cobalt powder, each ofwhich was a commercially available reagent, were mixed together at apredetermined ratio, and the mixture thus obtained was placed in acontainer made of copper, followed by sintering at 700° C. for 8 hoursin an nitrogen atmosphere. As a result, a blackish graylithium-containing complex nitride was produced as a sintered material.The produced lithium-containing complex nitride was pulverized and usedin a powder form. It should be noted that the series of steps, startingfrom mixing of the starting materials to pulverization of the sinteredmaterial, were performed in a nitrogen atmosphere of high purity havingthe total concentration of not more than 100 ppm of oxygen and water.The X-ray diffraction pattern of the obtained lithium-containing complexnitride powder was a hexagonal pattern as in the case of lithiumnitride, showing no impurity peak.

[0065] The obtained Li_(2.5)Co_(0.5)N powder was used to fabricate amodel cell similar to the one described above having a counter electrodecomprising metallic lithium, and the charge/discharge behavior andcapacity were confirmed. Herein, since nitrides react with water, theSBR dissolved in a dehydrated organic solvent was used.

[0066] The Li_(2.5)Co_(0.5)N desorbed Li ions in an amount correspondingto about 750 mAh/g during the initial discharge, and it also absorbed Liions in an amount corresponding to about 750 mAh/g during the subsequentcharge, having almost no irreversible capacity. Thereafter, the chargingand discharging continued to proceed with the reversible capacitymaintained at about 750 mAh/g.

[0067] Next, a negative electrode comprising the above-describedamorphous carbon powder and Li_(2.5)Co_(0.5)N powder was formed, andthis was used to fabricate a button-type test cell as shown in FIG. 1.Then, the cycle characteristic of the test cell was evaluated.

[0068] In FIG. 1, numeral 1 denotes a sealing plate made of stainlesssteel serving also as a negative electrode terminal. To the innersurface of the sealing plate, a nickel mesh 2 is welded, and the nickelmesh 2 is attached by pressure onto a copper foil serving as the currentcollector of a negative electrode 3. The negative electrode 3 was formedby applying a mixture containing the amorphous carbon andLi_(2.5)Co_(0.5)N to the copper foil.

[0069] Numeral 4 denotes a positive electrode case made of stainlesssteel, and a stainless steel mesh 5 is welded to the inner surface ofthe case. The stainless steel mesh 5 is attached by pressure onto analuminum foil serving as the current collector of a positive electrode6. The positive electrode 6 was formed by applying a mixture containingLiCoO₂ as an active material to the aluminum foil and punching it into adisc shape.

[0070] The surface of the positive electrode 6 is covered with a porousseparator 7 made of a polyethylene film. The inside of the positiveelectrode case 4 is filled with an electrolyte, and the separator 7 isswollen with the electrolyte.

[0071] On the periphery of the sealing plate 1, a gasket 8 is disposed,and the end portion of the opening of the positive electrode case 4 isbent towards the inside so as to clamp the gasket 8 thereby to seal thebattery. The gasket 8 ensures electrical insulation between the sealingplate 1 and the positive electrode case 4.

[0072] Herein, the negative electrode was fabricated in the followingmanner.

[0073] To 100 parts by weight of powder of the amorphous carbon, 40parts by weight of Li_(2.5)Co_(0.5)N powder, 20 parts by weight ofacetylene black as a conductive agent and 2 parts by weight of the SBRas a binder were added and mixed, and the mixture thus obtained wasdispersed in a dehydrated toluene to give a slurry-like mixture. Thismixture was applied, by using a doctor blade, onto a copper foil with athickness of 18 μm serving as a negative electrode current collector,which was dried and then rolled to form an negative electrode sheet.This negative electrode sheet was punched into a disc-shaped negativeelectrode plate with a diameter of 16 mm.

[0074] Additionally, the positive electrode was fabricated in thefollowing manner.

[0075] To 100 parts by weight of lithium cobaltate powder, 7 parts byweight of carbon powder as a conductive agent and 3 parts by weight ofpoly(vinylidene fluoride) resin as a binder were added, and the mixturethus obtained was dispersed in a dehydrated N-Methyl-pyrrolidinone togive a slurry-like mixture. This mixture was applied, by using a doctorblade, onto an aluminum foil with a thickness of 20 μm serving as apositive electrode current collector, which was dried and then rolled toform a positive electrode sheet. This positive electrode sheet waspunched into a disc-shaped positive electrode plate with a diameter of15 mm.

[0076] Additionally, the electrolyte was prepared by dissolving LiPF₆ ata concentration of 1 mol/L, in a mixed solvent containing ethylenecarbonate and ethylmethyl carbonate at a volume ratio of 1:1.

[0077] 0.5 g of lithium cobaltate as the positive electrode activematerial and 0.12 g of the total amount of the amorphous carbon andLi_(2.5)Co_(0.5)N as the negative electrode active materials werecontained in the test cell.

[0078] Charging/discharging test was conducted on the produced test cellat a constant current of 1 mA, with the end-of-charge voltage set at 4.1V and the end-of-discharge voltage at 2.0 V.

[0079]FIG. 2 shows: (A) the relation between the charge voltage andcapacity at 1st cycle; (B) the relation between the discharge voltageand capacity at 1st cycle; (C) the relation between the charge voltageand capacity at 5th cycle; and (D) the relation between the dischargevoltage and capacity at 5th cycle, of the test cell.

[0080] It should be noted that the average discharge voltage was about3.2 V and the discharge capacity was about 60 mAh. Although thecharging/discharging of this battery was started with charging, there isalmost no difference between the charge capacity and the dischargecapacity at 1st cycle in FIG. 2, indicating that the irreversiblecapacity of the amorphous carbon was effectively compensated for byLi_(2.5)Co_(0.5)N. Additionally, since there is almost no differencebetween the charge capacity and discharge capacity at 1st cycle and thecharge capacity and discharge capacity at 5th cycle, it is shown thatthe test cell had excellent cycle characteristic.

[0081] In FIG. 2, the charge/discharge voltage behavior at 1st cycle isdifferent from the charge/discharge voltage behavior at 5th cycle. Atand after 2nd cycle, the charge/discharge voltage behavior issubstantially the same as that at 5th cycle, and only the initialcharge/discharge exhibits a peculiar charge/discharge behavior. This isattributed to the fact that Li_(2.5)Co_(0.5)N did not participate in theinitial charge since the reaction potential of the amorphous carbon waslower than the reaction potential of Li_(2.5)Co_(0.5)N, and thatLi_(2.5)Co_(0.5)N transformed into amorphous during the initialdischarge to change the voltage profile.

[0082] Next, the capacity density of the negative electrode activematerial per weight in this battery was estimated.

[0083] The capacity density of lithium cobaltate in the positiveelectrode was about 120 mAh/g, which was the same as the theoreticalvalue. Further, the battery was limited by the positive electrode duringthe operation. Accordingly, the total capacity of the amorphous carbonand Li_(2.5)Co_(0.5)N, each of which was contained in the negativeelectrode, was not less than the capacity of the positive electrode.

[0084] From the above, since 0.5 g of lithium cobaltate and 0.12 g ofthe total amount of the amorphous carbon and Li_(2.5)Co_(0.5)N werecontained in the test cell, the capacity density of the negativeelectrode was estimated to be not less than 500 mAh/g.

[0085] In view of the fact that carbon materials in the negativeelectrodes of currently commercialized lithium ion batteries have acapacity density of about 300 to 370 mAh/g, the battery of the presentinvention has a negative electrode with an extremely high capacity.

[0086] Next, test cells were fabricated in the same manner as describedabove except that the value of X and the amount were changed for thelithium-containing complex nitride Li_(3-X)Co_(X)N. Herein, for eachbattery, the amount of lithium cobaltate contained in the positiveelectrode was adjusted to 0.5 g and the total amount of the amorphouscarbon and Li_(3-X)Co_(X)N contained in the negative electrode wasadjusted to 0.12 g. The discharge capacities (mAh) at 5th cycle obtainedfrom the respective batteries are shown in Table 1. TABLE 1 Amount ofnitride per 100 parts by weight of amorphous carbon Value (part byweight) of X 0 20 30 33 40 50 70 80 100 120 150 180 0.2 32 42 46 47 4952 56 56 55 55 53 43 0.3 32 51 58 60 60 60 60 57 55 55 53 41 0.4 32 5360 60 60 60 60 57 55 55 53 44 0.5 32 52 59 60 60 60 60 57 55 55 53 420.55 32 51 58 60 60 60 60 57 55 55 53 43 0.6 32 45 50 51 54 58 60 57 5454 53 39

[0087] In Table 1, the batteries using the negative electrodescontaining no lithium-containing complex nitride or containing the samein a small amount were deprived of Li ions owing to the irreversiblecapacity of the amorphous carbon, so that the discharge capacities werelower than the theoretical capacities. Conversely, in the case of thebatteries using the negative electrode containing the lithium-containingcomplex nitride in a large amount, the amount of the nitride that didnot participate in the charge/discharge increased to cause the batteriesto be limited by the negative electrode, so that the dischargecapacities were lower than the theoretical capacities. Although thesebatteries had higher capacity as compared to conventional ones, thepresent invention produced a particularly remarkable effect when thenegative electrode contained a certain amount or more of the nitride, orwhen the battery was limited by the positive electrode.

[0088] According to Table 1, when the negative electrode contained 20 to150 parts by weight, more preferably 33 to 70 parts by weight ofLi_(3-X)Co_(X)N (0.3≦X≦0.55) per 100 parts by weight of the amorphouscarbon, it was possible to yield a desirable battery performance.

EXAMPLE 2

[0089] A phenol resin was sintered at 1200° C. under an inert atmosphereto give an amorphous carbon. This was used to fabricate a model cellsimilar to that of Example 1 to evaluate the charge/dischargeperformance.

[0090] This amorphous carbon absorbed Li ions in an amount correspondingto about 800 mAh/g during the initial charge; however, it desorbed Liions in an amount corresponding to only about 460 mAh/g during thesubsequent discharge, having an irreversible capacity of about 340mAh/g.

[0091] With the use of this amorphous carbon and Li_(2.5)Co_(0.5)N, testcells similar to those of Example 1 were fabricated by employing 20, 33,70, 100, 120, 150 and 180 parts by weight of the nitride, respectively,per 100 parts by weight of the amorphous carbon, and the cycle testswere conducted.

[0092] The discharge capacities (mAh) at 5th cycle obtained from therespective batteries are shown in Table 2. TABLE 2 Amount of nitride per20 33 70 100 120 150 180 100 parts by weight of amorphous carbon (partby weight) Discharge capacity 51 60 60 60 60 59 51 (mAh)

[0093] According to Table 2, it was most suitable to include 33 to 150parts by weight of the nitride per 100 parts by weight of the amorphouscarbon in the negative electrode. It is considered that the use of aphenol resin as the starting material for this amorphous carbon causedthe laminated structure of carbon to become more isotropic as comparedto that of Example 1 and thereby provided good electronic conductivitybetween the active material particles. It is presumably for this reasonthat favorable results were obtained even in the cases where the amountof the nitride was relatively large.

EXAMPLE 3

[0094] In this example, a description is made on the case where a lowcrystalline carbon was used as a negative electrode active material. Thelow crystalline carbon was one prepared by sintering a petroleum pitchat 1100° C. under an inert atmosphere.

[0095] In general, sintering a petroleum pitch at a high temperature ofabout 3000° C. causes the crystal to grow to yield graphite. However, asin this example, sintering a petroleum pitch at a low temperature ofabout 1100° C. does not cause the crystal to grow, so that an extremelylow-crystalline carbon material is produced. In the X-ray diffractionpattern of the produced carbon material, a distinct peak as observed ina high crystalline graphite was not confirmed.

[0096] Next, this low crystalline carbon was used to fabricate a modelcell similar to that of Example 1 to evaluate the charge/dischargeperformance.

[0097] This low crystalline carbon absorbed Li ions in an amountcorresponding to about 775 mAh/g during the initial charge; however, itdesorbed Li ions in an amount corresponding to only about 610 mAh/gduring the subsequent discharge, having an irreversible capacity ofabout 165 mAh/g. Thereafter, the charging and discharging proceededreversibly.

[0098] Next, the low crystalline carbon and Li_(2.5)Co_(0.5)N were usedto fabricate a test cell similar to that of Example 1 to evaluate thecycle characteristic. The negative electrode comprising the lowcrystalline carbon and Li_(2.5)Co_(0.5)N was produced in the same manneras in Example 1 except that the low crystalline carbon was used in placeof the amorphous carbon.

[0099] It should be noted that 0.5 g of lithium cobaltate as thepositive electrode active material and 0.12 g of the total amount of thelow crystalline carbon and Li_(2.5)Co_(0.5)N as the negative electrodeactive materials were contained in the test cell.

[0100] Charging/discharging test was conducted on the test cell at aconstant current of 1 mA, with the end-of-charge voltage set at 4.1 Vand the end-of-discharge voltage at 2.0 V.

[0101]FIG. 3 shows: (E) the relation between the charge voltage andcapacity at 1st cycle; (F) the relation between the discharge voltageand capacity at 1st cycle; (G) the relation between the charge voltageand capacity at 5th cycle; and (H) the relation between the dischargevoltage and capacity at 5th cycle, of the test cell.

[0102] The average discharge voltage was about 3.1 V and the dischargecapacity was about 60 mAh. Although the charging/discharging of thisbattery was started with charging, there is almost no difference betweenthe charge capacity and discharge capacity at 1st cycle in FIG. 3,indicating that the irreversible capacity of the low crystalline carbonwas effectively compensated for by Li_(2.5)Co_(0.5)N. Additionally,since there is almost no difference between the charge capacity anddischarge capacity at 1st cycle and the charge capacity and dischargecapacity at 5th cycle, it is shown that the test cell had excellentcycle characteristic.

[0103] It should be noted that a change of the voltage profile as shownin FIG. 2 due to the transformation of Li_(2.5)Co_(0.5)N into amorphouswas also observed in this case.

[0104] The capacity density of the negative electrode per weight of theactive material in this battery was estimated in the same manner as inExample 1. As with Example 1, the battery of the present example wasalso limited by the positive electrode during the operation. Therefore,the low crystalline carbon and Li_(2.5)Co_(0.5)N were estimated to havea capacity density of not less than 500 mAh/g.

[0105] Next, test cells were fabricated in the same manner as describedabove except that the value of X and the amount were changed for thelithium-containing complex nitride Li_(3-X)Co_(X)N. Herein, for eachbattery, the amount of lithium cobaltate contained in the positiveelectrode was adjusted to 0.5 g, the total amount of the low crystallinecarbon and Li_(3-X)Co_(X)N contained in the negative electrode wasadjusted to 0.12 g. The discharge capacities (mAh) at 5th cycle obtainedfrom the respective batteries are shown in Table 3. TABLE 3 Amount ofnitride per 100 parts by weight of low Value crystalline carbon (part byweight) of X 0 20 25 30 50 55 60 100 120 150 180 0.2 40 48 50 52 57 5857 55 55 53 44 0.3 40 57 60 60 60 60 58 56 55 53 42 0.4 40 59 60 60 6060 58 56 55 54 43 0.5 40 58 60 60 60 60 58 56 55 54 41 0.55 40 57 60 6060 60 58 56 55 54 42 0.6 40 51 54 56 60 60 58 54 54 53 41

[0106] In Table 3, the batteries using the negative electrodescontaining no lithium-containing complex nitride or containing the samein a small amount were deprived of Li ions owing to the irreversiblecapacity of the low crystalline carbon, so that the discharge capacitieswere lower than the theoretical capacities. Conversely, in the case ofthe batteries using the negative electrode containing thelithium-containing complex nitride in a large amount, the amount of thenitride that did not participate in the charge/discharge increased tocause the battery to be limited by the negative electrode, so that thedischarge capacities were lower than the theoretical capacities.Although these batteries had higher capacity as compared to conventionalbatteries, the present invention produced a particularly remarkableeffect when the negative electrode contained a certain amount or more ofthe nitride, or when the battery was limited by the positive electrode.

[0107] According to Table 3, when the negative electrode contained 20 to150 parts by weight, more preferably 20 to 55 parts by weight ofLi_(3-X)Co_(X)N (0.3≦X≦0.55) per 100 parts by weight of the lowcrystalline carbon, it was possible to yield a desirable batteryperformance.

EXAMPLE 4

[0108] A mesophase pitch produced by polymerizing naphthalene wassintered for one hour at 700° C. under an inert atmosphere to give a lowcrystalline carbon. This was used to fabricate a model cell similar tothat of Example 1 to evaluate the charge/discharge performance. Thiscrystalline carbon absorbed Li ions in an amount corresponding to about970 mAh/g during the initial charge; however, it desorbed Li ions in anamount corresponding to only about 660 mAh/g during the subsequentdischarge, having an irreversible capacity of about 310 mAh/g. With theuse of this low crystalline carbon and Li_(2.5)Co_(0.5)N, test cellssimilar to those of Example 1 were fabricated by employing 20, 25, 70,100, 120, 150 and 180 parts by weight of the nitride, respectively, per100 parts by weight the low crystalline carbon, and the cycle tests wereconducted. The discharge capacities (mAh) at 5th cycle obtained from therespective batteries are shown in Table 4. TABLE 4 Amount of nitride per20 25 70 100 120 150 180 100 parts by weight of low crystalline carbon(part by weight) Discharge capacity 45 58 60 60 60 60 53 (mAh)

[0109] Table 4 demonstrates that it was most suitable to include 25 to150 parts by weight of the nitride per 100 parts by weight of the lowcrystalline carbon, in the negative electrode. It is considered that,the use of a mesophase pitch as the starting material for this lowcrystalline carbon caused the laminated structure of carbon to becomemore isotropic as compared to that of Example 3 and thereby providedgood electronic conductivity between the active material particles. Itis presumably for this reason that the favorable results were obtainedeven in the cases where the amount of the nitride was relatively large.

EXAMPLE 5

[0110] In this example, a description is made on the case where afibrous low crystalline carbon was used as a negative electrode activematerial. The fibrous low crystalline carbon was one prepared bysoftening a mesophase pitch at 200° C. and then transforming it into afibrous form through a spinning process, followed by a curing processfor 12 hours at 150° C. in air, and thereafter, sintering it for 5 hoursat 700° C. under an inert atmosphere.

[0111] Then, the obtained sintered material was pulverized and observedwith SEM, and a fibrous carbon having a mean fiber diameter of 10 μm anda mean fiber length of 100 μm was observed.

[0112] Next, this fibrous carbon was used to fabricate a model cellsimilar to that of Example 1 to evaluate the charge/dischargeperformance.

[0113] This fibrous carbon absorbed Li ions in an amount correspondingto about 1000 mAh/g during the initial charge; however, it desorbed Liions in an amount corresponding to only about 700 mAh/g during thesubsequent discharge, having an irreversible capacity of about 300mAh/g. Thereafter, the charging and discharging proceeded reversibly.

[0114] Next, the fibrous carbon and Li_(2.5)Co_(0.5)N were used tofabricate a test cell similar to that of Example 1 to evaluate the cyclecharacteristic.

[0115] The negative electrode comprising the fibrous carbon andLi_(2.5)Co_(0.5)N was fabricated in the same manner as in Example 1,except for using the fibrous carbon in place of amorphous carbon andadding to 100 parts by weight of the fibrous carbon powder, 55 parts byweight of Li_(2.5)Co_(0.5)N powder, 20 parts by weight of acetyleneblack as a conductive agent and the SBR as a binder.

[0116] It should be noted that 0.5 g of lithium cobaltate and 0.12 g ofthe total amount of the fibrous carbon and Li_(2.5)Co_(0.5)N werecontained in the test cell.

[0117] Charging/discharging test was conducted on the produced test cell(hereinafter, referred to as “Battery A”) at a constant current of 1 mA,with the end-of-charge voltage set at 4.1 V and the end-of-dischargevoltage at 2.0 V. The average discharge voltage was about 3.1 V and thedischarge capacity was about 60 mAh.

[0118] Although the charging/discharging of this battery was startedwith charging, there is almost no difference between the charge capacityand discharge capacity at 1st cycle, indicating that the irreversiblecapacity of the fibrous carbon was effectively compensated for byLi_(2.5)Co_(0.5)N. Additionally, since there was almost no differencebetween the charge and discharge capacities at 1st cycle and those at30th cycle, it was shown that this test cell also had excellent cyclecharacteristic.

[0119] Next, Batteries B, C and D were fabricated in the same manner asthat used in Battery A except that the amount of acetylene black as theconductive agent per 100 parts by weight of the fibrous carbon waschanged to 10, 5 and 1 part by weight, respectively.

[0120] The discharge capacities (mAh) of the respective batteries at 1stcycle and 30th cycle are shown in Table 5. TABLE 5 Amount of DischargeDischarge acetylene black capacity at 1st capacity at 30th (part byweight) cycle (mAh) cycle (mAh) Battery A 20 60 60 Battery B 10 66 65Battery C 5 68 67 Battery D 1 72 70

[0121] In Table 5, in the case where the fibrous carbon was used as thelow crystalline carbon, no significant reduction was observed in thecycle characteristic even when the amount of acetylene black as theconductive agent was reduced. The reason is presumably that thereduction in electronic conductivity of the nitride was suppressed sincethe fibrous carbon functioned as the conductive agent, in place ofacetylene black. Further, the reduction of acetylene black resulted inthe increase of the content proportion of the nitride and lowcrystalline carbon in the electrode plate, thereby also achieving anadvantage of yielding a high capacity battery.

[0122] From the above, it was found that the use of a fibrous carbon asa low crystalline carbon could yield a battery having high capacity andexcellent cycle characteristic.

[0123] Industrial Applicability

[0124] As described above, it is possible to maximize the utilization ofa large charge/discharge capacity inherently possessed by an amorphouscarbon or low crystalline carbon by compensating for the irreversiblecapacity of the amorphous carbon or low crystalline carbon by alithium-containing complex nitride having high capacity, therebyproviding a non-aqueous electrolyte secondary battery having highcapacity and excellent cycle reversibility.

1. A negative electrode for a non-aqueous electrolyte secondary batterycomprising: 100 parts by weight of a carbon material having irreversiblecapacity during the initial charge and discharge; and 20 to 150 parts byweight of a lithium-containing complex nitride represented by thegeneral formula Li_(3-X)M_(X)N wherein M is at least one selected fromthe group consisting of Co, Ni, Mn and Cu, and wherein 0.2≦X≦0.8.
 2. Thenegative electrode for a non-aqueous electrolyte secondary battery inaccordance with claim 1, wherein said lithium-containing complex nitrideis Li_(3-X)Co_(X)N wherein 0.2≦X≦0.55.
 3. The negative electrode for anon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said carbon material is a low crystalline carbon.
 4. Thenegative electrode for a non-aqueous electrolyte secondary battery inaccordance with claim 3, wherein said low crystalline carbon is in afibrous form having a mean fiber diameter of 1 to 50 μm and a mean fiberlength of 10 to 200 μm.
 5. The negative electrode for a non-aqueouselectrolyte secondary battery in accordance with claim 1, wherein saidcarbon material is an amorphous carbon.
 6. A non-aqueous electrolytesecondary battery comprising: a positive electrode comprising alithium-containing complex oxide capable of absorbing and desorbinglithium ion; the negative electrode in accordance with claim 1; and anon-aqueous electrolyte interposed between said positive electrode andnegative electrode.